I wonder if the reward values have changed since the beta? I'm sure it is much harder to find anything now than it would have been back then, assuming they got a good turnout from really experienced people and 7-8 months of headstart.
"We are using a simple severity ranking scheme: Low - Medium - High - Critical. Rewards range from $100 up to $5000 and are determined at our discretion based on a number of factors. For example, if you find a reflected XSS that is only possible in Opera, and Opera is only 1.64% of our traffic, then the severity and reward will be lower. But a persistent XSS that works in Chrome, at 59.53% of our traffic, will earn a much larger reward."